Troy Davis was executed this evening for the murder of an off-duty policeman after the U.S. Supreme Court denied a last-minute stay of execution amid widespread public doubts about his guilt.

Davis, 42, died at 11:08 p.m. ET, according to a Georgia Department of Corrections official. His death by lethal injection came after an approximately four-hour delay for legal review.

Eyewitnesses described the mood in the execution chamber as "somber" as Davis was wheeled in strapped to a gurney. He declared his innocence a final time in the 1989 murder as witnesses and relatives of the victim -- off-duty Savannah, Ga., policeman Mark MacPhail -- looked on.

"I'd like to address the MacPhail family," Davis said, according to The Associated Press. "Let you know, despite the situation you are in, I'm not the one who personally killed your son, your father, your brother. I am innocent.

"The incident that happened that night is not my fault," he added. "I did not have a gun. All I can ask ... is that you look deeper into this case so that you really can finally see the truth.

"I ask my family and friends to continue to fight this fight," he said. "For those about to take my life, God have mercy on your souls. And may God bless your souls."

Witnesses said Davis' eyes fluttered as he received his first injection and lost consciousness, and that the entire process of lethal injection lasted about 15 minutes.

"Justice has been served for Officer Mark MacPhail and his family," state Attorney General Sam Olens said in a statement.

Joan MacPhail-Harris, the widow of Mark MacPhail, told The Associated Press that "it's a time for healing" now that Davis' execution has occurred, that she saw "nothing to rejoice" over in Davis' death and that she was praying for his family.

"I will grieve for the Davis family because now they're going to understand our pain and our hurt," she told the AP in a telephone interview from Jackson.

"I'm kind of numb. I can't believe that it's really happened," MacPhail's mother, Anneliese MacPhail, told the AP in a telephone interview from her home in Columbus, Ga. "All the feelings of relief and peace I've been waiting for all these years, they will come later. I certainly do want some peace."

Members of the MacPhail family are convinced Davis was guilty, but many other observers are not.

Davis had his execution stayed four times over the course of his 22 years on death row, but multiple legal appeals during that time failed to prove his innocence.

Public support grew for Davis based on the recanted testimony of seven witnesses from his trial and the possible confession of another suspect, which his defense team claimed cast too much doubt on Davis' guilt to follow through with an execution.

A growing tide of celebrities, politicians and social media users called for the execution to be delayed because of "too much doubt" present in his case.

Up to and beyond the moments of execution, the criticism continued. "Strange Fruit," a classic song about lynching, trended on Twitter, celebrities tweeted their disapproval and, after it was over, an Amnesty International official released a written statement condemning the execution.

"Killing a man under this enormous cloud of doubt is horrific and amounts to a catastrophic failure of the justice system," said Larry Cox, executive director of Amnesty International AIUSA. "Our hearts are heavy, but we have not lost our spirit of defiance. Millions of people around the world now know of Troy Davis and see the fallibility of the U.S. justice system."

The execution was delayed more than four hours as the U.S. Supreme Court weighed last-minute arguments from Davis' legal team and the state of Georgia over whether his execution should be blocked.

The court's decision to deny the stay came without comment after 10 p.m. ET.

Troy Davis Backers in Frantic Last Minute Efforts to Stop Execution

At 7:05 p.m. ET, five minutes after his scheduled death, Davis' supporters erupted in cheers, hugs and tears outside the jail in Jackson, Ga., as supporters believed Davis had been saved from the death penalty. But Davis was granted only a temporary reprieve as the Supreme Court considered the decision.

At a protest in front of the White House today, at least 12 Howard University students were arrested for failing to move off the White House sidewalk, according to ABC News affiliate WJLA. The protest there was expected to last until 7 p.m.

A flurry of messages on Twitter using the hashtags #TroyDavis and #TooMuchDoubt showed thousands of supporters of Davis were intent on flooding the Jackson District Attorney's Office, Georgia Judge Penny Freezeman's office, and the U.S. Attorney General's Office with phone calls and e-mails to beg for a stay on the execution.

Some users accused Twitter of blocking the topic from trending on Tuesday, though a representative from Twitter told ABC News there was no such action taken. The hashtags were trending today in cities around the U.S. as well as Germany, the U.K., Sweden and France. Many tweets called the case a symbol of a return to Jim Crow laws and racial inequalities in the justice system.

Big Boi, a member of the group Outkast, tweeted to his followers to go to the Georgia state prison in Jackson to protest the decision. The Roots' Questlove tweeted a similar message.

The NAACP and the Revs. Al Sharpton and Jesse Jackson held a news conference today calling for the execution to be halted.

Amnesty International, which has been fighting on behalf of Davis, encouraged supporters to attend a vigil at the church across the street from the prison at 5:30 p.m. and a protest at 6 p.m., and asked participants to wear a black armband and write on it, "Not in my name!"

Wendy Gozen Brown, a spokeswoman for Amnesty International, said that Davis would want the protests to remain peaceful.

"In this type of situation, there's always the potential for it to go awry, with certain groups, angry rhetoric," Brown said. "But Troy Davis would want people to keep fighting peacefully, for him and for, as he would put it, all of the other Troy Davises out there."

Others who have voiced support for Davis include former President Jimmy Carter, the pope and a former FBI director.

Davis's execution was stayed four times for appeals since his conviction in 1991, and the Supreme Court gave him a rare chance to prove his innocence last year, but rejected his plea.

A Georgia board of pardons and paroles rejected Davis's plea for clemency on Tuesday.

ABC News' Arianne de Vogue and Steve Osunsami and The Associated Press contributed to this report.